1. Field of the Invention
The invention relates generally to methods and apparatus for preparing and delivering a self curing bone cement formed as a polymeric reaction product after mixing a powdered polymer component with a liquid monomer component.
More particularly, the invention relates to methods and apparatus for preparing and delivering bone cement by combining a powdered polymer component stored in an evacuated mixing chamber with a liquid monomer component stored in a pressurized chamber (e.g., under atmospheric pressure), where the vacuum is used to draw the liquid into the mixing chamber at a desired point in time.
According to one specific aspect of the invention, the liquid monomer is introduced into the evacuated mixing chamber by control means which causes the stored powder to be "substantially wet". As used herein the term "substantially wet" is defined to mean all of the powdered polymer component is wet to at least some degree and is otherwise devoid of powder pockets.
The control means, according to a preferred embodiment of the invention, stages the release of the liquid monomer into the vacuum packed powder chamber, under the effect of the vacuum, in a timed sequence using a plurality of monomer distribution channels.
The distribution channels, which include liquid component diffusion means, insure the substantial wetting of the stored powder by delivering the liquid in parallel to different portions of the evacuated mixing chamber. The substantial wetting of the powder is designed to ultimately insure a homogeneous bone cement mix.
2. Brief Description of the Prior Art
In many orthopedic surgical procedures it is necessary to employ a bonding material to set implants such as pins and artificial joints in bone. The demand for bone cement for such applications has increased in spite of the growth of cementless implant technology. This is because bone cement has distinct indications and a surgeon will usually specify a cement based upon its tensile strength and workability.
The cement employed for surgical purposes is generally a polymeric material which is prepared by copolymerization of its components as needed. Because of the necessity for a fairly quick setting material the cement is almost universally prepared by a surgical assistant during the course of the operation in the sterile field of the operating room. The preparation of the cement involves mixing the aforementioned components in a suitable vessel.
The cement is usually a (meth)acrylic material which is a reaction product of a monomer and a polymer, e.g., methylmethacrylate monomer and polymethylmethacrylate or methylmethacrylate-styrene copolymer. In order to provide a cement which has the desired properties and which has the desired fixation of the implants, it is necessary that the compounds be uniformly and thoroughly mixed so that a homogeneous reaction product is produced.
Many systems exist for the preparation and delivery of a polymethylmetacrylate bone cement formed from powdered polymer and liquid monomer components. However, the known systems have various shortcomings, to be discussed hereinafter, which make a quality cement (in terms of strength, durability, etc.), difficult to prepare.
Careful mixing is required to maximize the amount of powder that is wet and to keep out (and/or remove) as much air as possible from the mixture. Failure to achieve these two objectives can result in unpolymerized powder contaminating and/or air bubbles weakening the final reaction product cement mixture. These contaminants have the potential for compromising the long term performance of the cement and any associated implant.
Furthermore, the known processes for mixing polymethylmetacrylate cement are often complicated by the need to deal with noxious fumes produced when the aforementioned components are mixed, and because the useful working time for the cement (time before it hardens) is short.
In spite of the aforementioned difficulties, manufacturers have made conscientious efforts to improve systems for preparing and delivering bone cement by, for example, using component mixing chambers connected to vacuum pumps to filter fumes; providing mixing paddles to try and produce a more uniform cement mixture; using compression techniques to "squeeze" out air from the mixture; using dispensing guns to eject premixed material loaded into the gun, and so on.
These known systems are inherently problematic from several reasons. Vacuum tubes must be hooked up; mixing paddles must be engaged and do not insure uniform saturation of the powder component; the noxious fumes must be drawn away from the mixing site; and any premixed cement must actually be placed into the dispensing apparatus.
Furthermore, the two components themselves must be removed from separate packaging and be placed into a mixing chamber. The separate packaging is required since the monomer and cement powder can not be sterilized in the same manner. The liquid component requires aseptic packaging, while the powder component must be gamma sterilized.
The two sterile packages are typically comprised of different structures (usually a glass vial for the liquid and a plastic packet for the powder). Furthermore, the two components have different consistences, densities and mixing properties. All of the aforementioned factors make the critical mixing sequence difficult.
The prior art has attempted to address many of the aforementioned problems by, for example, utilizing compressive force to remove air from a cement mixture as the cement is being formed; by using a vacuum packed solid component chamber to draw the liquid component into spaces between the solid particles in an attempt to achieve a more uniform component mixture; by filtering fumes created during the reaction process, as will be explained hereinafter with reference to several issued patents which exemplify the state of the art.
In particular, Gunnarsson, in U.S. Pat. No. 4,758,096; Draenert, in U.S. Pat. No. 4,996,601; Tepic, in U.S. Pat. No. 5,051,482; Chan, in U.S. Pat. Nos. 4,973,168 and 5,100,241; and Kindt-Larsen et al., in U.S. Pat. No. 5,114,240 (and corresponding Danish Patent Number 2355/89), are referred to hereinafter to illustrate the present state of the bone cement preparation and delivery art.
These patents are broadly directed to two types of systems. Systems that use a vacuum for the mixing process and systems that use a compressive force to mix the bone cement components.
In particular, Gunnarsson, in U.S. Pat. No. 4,758,096, issued Jul. 19, 1988, is a background reference which teaches apparatus for mixing bone cement components in a vacuum. The reference cites minimizing the potential for air remaining in the reaction product as the principal advantage of mixing the cement in an evacuated container.
The Gunnarsson reference requires that the mixing container be connected to an external vacuum source after the components of the bone cement are first placed into the mixing container. Once under a vacuum, the components are manually agitated to create the mix.
Such a system, although reducing the amount of air in the mixture by performing the mixing process in a vacuum, does little to insure that all of the powder is wet before the reaction process begins. As indicated hereinabove, failure to address this problem can result in inclusions being formed in the finished cement product as a result of unpolymerized powder being included in the mix.
Furthermore, the teachings of Gunnarsson require the use of a separate vacuum pump to evacuate the mixing chamber (once the components are placed therein), with the vacuum pump requiring manual hookup and a start up delay before the mixing process can begin. This delay takes valuable time away from being able to achieve a thorough mix since the reactive components have already been placed together (and are reacting to one another) in the mixing chamber.
Further examples of techniques for mixing bone cement components in a vacuum are described by Tepic, in U.S. Pat. No. 5,051,482; and Chan, in U.S. Pat. No. 4,973,168 and 5,100,241.
Tepic, in U.S. Pat. No. 5,051,482, issued Sep. 24, 1991, describes methods and apparatus for preparing a self-curing two-component powder liquid bone cement using a vacuum packed powder chamber into which monomer is introduced. The Tepic system was designed to eliminate the need for mechanical stirring as, for example, required by Gunnarsson.
Nevertheless, Tepic, like Gunnarsson, did not solve or even address the problem of insuring that all of the powder is thoroughly saturated with the liquid bone cement component before the reaction process begins.
In particular, utilizing the teachings of Tepic, the powder polymer can not be sufficiently infused with liquid monomer so as to produce a proper cement mixture. The initial portion of monomer introduced into the powder starts to catalyze with the powder thereby creating a blockage for the remaining monomer.
As a result, the balance of the monomer can not find its way past such an obstruction, or "clog", thereby depriving the rest of the powder of any monomer whatsoever causing substantial amounts of powder to remained dry and unmixed. Similar shortcomings are inherent in all known vacuum mix systems, including the Chan patents discussed hereinafter.
Chan, in U.S. Pat. No. 5,100,241, issued Mar. 31, 1992, and U.S. Pat. No. 4,973,168, teaches a two-component bone cement mixing system comprising a cartridge mixer having an interior volume containing a first predetermined quantity of a free-flowing, powdery, solid bone cement component under vacuum pressure, an ampoule containing a second predetermined quantity of a liquid bone cement component, and a fluid transfer mechanism for fluidically connecting the cartridge mixer and ampoule.
The two-component bone cement mixing system described by Chan allows in vacuo mixing of liquid monomeric and solid polymeric bone cement components without air being incorporated into the mixture and prevents the passage of air into the cartridge mixer during and/or after monomeric transfer. The mixing process itself is carried out by causing reciprocating motion of a mixing element within the cartridge mixer.
Chan however, like the other references cited hereinabove, fails to teach, claim or even suggest methods and apparatus which insure that all of the powder is thoroughly saturated with the liquid bone cement component before the reaction process begins.
Furthermore, Chan's system for admixing bone cement components under vacuum pressure contemplates using a predetermined degree of chilling to control the rate of hardening of the cement, and subsequent pressurizing of the admixture to help inhibit entrainment of gaseous materials in the cement mix.
Accordingly, the system contemplated by Chan not only fails to solve the problem of substantially (thoroughly) wetting the powder, but is unduly complicated to operate.
Draenert, in U.S. Pat. No. 4,996,601, issued Oct. 30, 1990, describes apparatus for mixing and applying bone cement using an evacuatable bone cement syringe. The syringe system described includes a container for receiving the bone cement prior to its application, a pressure generating apparatus for precompressing the bone cement in the container; and a bell, comprising a vacuum tube, placed over the container (and held by a flange) so that the gases which escape during the process of decompression can be sucked off to reduce the porosity of the bone cement being applied.
It should be noted that the vacuum used in the Draenert system is for outgassing purposes (not for mixing the bone cement components as described by the other references cited hereinbefore), and that Draenert uses compressive force to squeeze air out from the cement mixture. Furthermore, Draenert completely fails to teach, claim or even suggest how to solve the aforementioned uniform mixing (thorough powder saturation) problem recited hereinbefore.
Finally, the present state of the art can be more fully appreciated with reference to an alternate approach to using a vacuum to mix bone cement described by Kindt-Larsen et al., in U.S. Pat. No. 5,114,240, issued May 19, 1992 (also described in corresponding Danish Patent Number 2355/89); and with further reference to copending patent application Ser. No. 08/076,556, entitled "Methods and Apparatus For Preparing and Delivering Bone Cement", filed Jun. 11, 1993 and assigned to the same assignee as the present invention. The copending patent application is hereby incorporated by reference.
According to Kindt-Larsen et al., a paste like material is provided by a mixing device that includes a first cylinder for containing a powdered component and a second cylinder for containing a liquid component. The first cylinder has a closed first end and an opposite second end provided with venting means. The second cylinder has a closed first end and an opposite second end sealingly receiving the closed first end of the first cylinder together in a piston-like manner.
The device also includes means for communicating between the inner spaces of the first and second cylinders through the closed first end of the first cylinder, whereby liquid from the second cylinder is injected into interstices defined between the powdered component contained in the first cylinder for providing a paste-like cementitious material when the first cylinder is forced into the second cylinder.
The Kinde-Larsen et al. device operates by using pressure developed by a caulking gun to force the isolated powder and liquid components together. In particular, liquid monomer is forced through the stored powder removing air from the powder compartment. The air and gasses developed during the polymerization process are vented through an activated carbon filter. Both the air and fumes are expelled by compressing the two components together.
As indicated hereinbefore, systems like the one disclosed in the Kindt-Larsen et al. reference are prone to produce cement having inclusions caused by entrapped air bubbles not removed by the applied compressive force and inclusions caused by unpolymerized powder. Furthermore, the type of system described by Kindt-Larsen et al., requires fumes be expelled and filtered while the mixing process takes place; requiring a charcoal filter to absorb the fumes and requiring the use of a relatively unreliable force (a manually applied compressive force), to mix the monomer and powder, and to squeeze out gasses.
The incorporated patent application relates to methods and apparatus for preparing and delivering bone cement by simultaneously injecting powder and liquid components (previously sterilized and stored in separate component chambers), into an evacuated mixing chamber; uniformly mixing the components therein (to substantially wet the powder); and delivering the cement formed by the polymerization process, as needed in an operative setting, utilizing a single ready to use device.
The uniform mix is achieved by simultaneously subjecting the contents of the component chambers to the effects of the mixing chamber's vacuum via separate one way nozzles, interconnecting the component chambers to the mixing chamber, which cause the powder to be broadcast and liquid to be sprayed into the mixing chamber. The nozzles are designed and oriented in a predetermined manner (depending on the type and quantity of the constituent bone cement components being mixed), to cause the components to interact at a preselected distance within the mixing chamber, to assure uniform powder saturation and a thorough mix which yields a homogeneous reaction product.
Although producing a high quality cement mix, the "injection" technique taught in the incorporated copending patent application is a three chambered device (two storage chambers and one mixing chamber), and requires the use of the aforementioned specially designed nozzles to cause the components to interact at a preselected distance within the mixing chamber.
In view of the state of the art as illustrated by the aforementioned references, it would be desirable to provide methods and apparatus which reduce the potential for unpolymerized powder contaminating the cement mixture formed as a reaction product to the aforementioned powdered and liquid bone cement components.
To this end it would be desirable to provide methods and apparatus which produce a uniform powder/monomer mix, with the monomer wetting as much powder as possible before polymerization begins to thereby improve cement durability.
It would be further desirable to provide methods and apparatus which not only substantially wet the powder with the liquid monomer component, but which also keep, as well as remove, as much air as possible out of the resulting reaction product cement mixture to minimize the potential for air bubble inclusions in the reaction product to further increase cement durability.
Still further, it would be desirable to provide methods and apparatus which do not require the use of vacuum pumps, vacuum hoses, separate venting systems, filters, etc., to deal with the venting of noxious fumes and other gases produced when the aforementioned components are mixed.
Further yet, it would be desirable to provide methods and apparatus which enable bone cement to be prepared using a single ready to use two chamber device not requiring the addition of bone cement components, vacuum hookups, etc., which maintains the integrity of the sterile bone cement components throughout the cement preparation operation and which is designed to cooperate with a cement delivery unit to complete the cement preparation and delivery process.
It would also be desirable to provide methods and apparatus for preparing bone cement which do not require that the reactive components be brought together prior to performing the mixing operation per se; which utilize the effects of a vacuum to introduce the liquid component into an evacuated packed powder chamber in a manner designed to insure that the powder is substantially wet; and which uniformly mixes the powdered component and liquid component within the mixing chamber to thereby saturate the powdered component with the liquid component.